Dairy Process Engineering

Module 1. Evaporation

Lesson 1
BASIC PRINCIPLES OF EVAPORATORS

1.1 Introduction

Evaporation and vapouration are two processes in which simultaneous heat and mass transfer process occurs resulting into separation of vapour from a solution. Evaporation and vapourization occur where molecules obtain enough energy to escape as vapour from a solution. The rate of escape of the surface molecules depends primarily upon the temperature of the liquid, the temperature of the surroundings, the pressure above the liquid, surface area and rate of heat propagation to product.

1.2 Vapourization and Evaporation

Evaporation and vaporization are quite different from each other. The differences are shown in Table 1.1

Table 1.1 Differences between evaporation and vaporization

Evaporation and vapourization occur where molecules obtain enough energy to escape as vapour from a solution. The rate of escape of the surface molecules depends primarily upon the temperature of the liquid, the temperature of the surroundings, the pressure above the liquid, surface area and rate of heat propagation to product. In a closed container with air space above the liquid, evaporation will continue until the air is saturated with water molecules. Removal of water from a liquid product by evaporation is enhanced by adding heat and by removing the saturated air from above the liquid. This is done by removal of vapour from the space above the liquid surface and there by creating vacuum. The boiling point of solution due to dissolved solutes is higher than that of pure water and depends on the molecular weight of the solute. Vacuum is utilized to remove water from liquid/solids at lower temperatures to reduce damage to heat sensitive products which might decompose at higher temperatures.

In the dairy industry evaporation means the concentration of liquid milk products containing dissolved, emulsified or suspended constituents. During this process water is removed by boiling. This process is used in the dairy industry for manufacture of evaporated milk, condensed milk and traditional Indian Dairy products i.e. Kheer, Basundi, Khoa etc.

In milk condensing plant, milk is condensed by evaporating a part of its water content by using saturated steam. The milk is boiled under vacuum. As the milk boils, water vapour is formed. This vapour is utilized for heating the milk further in the next stage which is at a higher vacuum.

Modern dairy plants use evaporators to remove part of water from milk by boiling it under low pressure. The process of evaporation takes place at a maximum temperature of about 70 0C corresponding to an absolute pressure of 230 mm (9.0 inch) of mercury (Hg). Evaporation of milk under low pressure or vacuum is carried out in a specially designed plant. The plant design depends much on the characteristics of liquid milk during boiling at low pressure than any other factor. Some of the important properties of evaporating milk are as under.

i Concentration of solids (initial and final)

ii Foaming under vacuum

iii Heat sensitivity

iv Viscosity change

The engineering design of plant requires certain other factors which provide a suitable milk contact surface, cleaning without frequent dismantling, faster heat transfer and economy of steam/power used for operating the plant.

Following factors are important for evoparation process.

(i) Concentration: The initial and final concentration of solute in the solution should be considered. As the concentration increases, the boiling point rises.

(ii) Foaming: Few products have tendency to foam, which reduce heat transfer and there is difficulty in controlling level of liquid which ultimately increases product (entrainment) losses.

(iii) Heat sensitivity: Milk, like many other food products, is sensitive to high temperatures. If time of exposure is more, there will be severe damage to milk proteins.

(iv) Scale formation / Fouling: It is a common phenomenon of deposition of solids on the heat exchanger surface. However, the scale forming tendency can be very much reduced by maintaining reasonably low temperature difference and relatively clean and smooth heat transfer surface. The flow velocity of product has also significant effect. If scale formation starts, rate of heat transfer decreases and cleaning becoming more difficult.

(v) Materials of construction: Stainless steel is the most common metal for evaporators in the dairy and food industry. Other metals may be used in chemical evaporators. The factors like strength, toughness, weld-ability, non-toxicity, surface finish, cost etc. are important in the selection of material of construction.

(vi) Specific heat: It changes with concentration of solution. More heat is required to be supplied at high specific heat values.

(vii) Gas liberation: Few products liberate gases when heated under boiling pressures.

(viii) Toxicity: The gases liberated in few cases may be toxic and should be handle carefully.

(ix) Viscosity: There is increase in viscosity of solution during evaporation which increases time of contact and hence chances of burning or damage the product.

(x) Capacity: It is expressed as the amount of water evaporated per hour. It depends on the surface area of heat transfer, temperature difference and the overall heat transfer co-efficient.

(xi) Economy: It is based on the amount of water evaporated per kg of steam used. It increases with number of effects.